Error Recognition in Interferometric Imaging

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Error Recognition in Interferometric Imaging
A Lecture Presented at the 8th Synthesis Imaging
Summer School
Socorro, New Mexico
20 June 2002
Steven T. Myers (NRAO-Socorro)
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Goals

• Locate the causes of possible errors and defects
• Relate visibility errors to image defects
• Learn methods of diagnosing common problems
• Find solutions for correction of errors
• Figure out who to blame when you cant fix it!

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Where Are Errors Introduced?

• far field (atmosphere, ionosphere, beyond)
• near field (atmosphere, ionosphere)
• antenna frontend (optics, receiver)
• antenna backend (amplifiers, digitizers, etc.)
• baseline (correlator)
• post-correlation (computer, software, user)

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Interferometer Signal Block Diagram
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Topography of the uv Plane

• the sky has spherical geometry
• spherical harmonics form complete basis
• multipole expansion in l,m
• at small angles this is Fourier transform in u,v
• uv plane momentum space
• points in uv plane plane waves on sky
• effects in one domain mirror the other
• uncertainty principle localized ? broadened
• truncation ? convolution

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The Fourier Planes

• image plane and aperture (uv) planes are conjugate

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Mosaicing

• synthesizing wider field narrows uv plane
  resolution

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Image or Aperture Plane?

• errors obey Fourier transform relations
• narrow features transform to wide features (and
  visa versa)
• symmetries important (real/imag, odd/even,
  point/line/ring)
• the transform of a serious error may not be
  serious!
• effects are diluted by the number of other
  samples
• watch out for short scans or effect on
  calibration solutions
• some errors more obvious in particular domain
• switch between image and uv planes

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The 2D Fourier Transform

• x,y (radians) in tangent plane relative to phase
  center
• spatial frequency u,v (wavelengths)
• adopt the sign convention of Bracewell

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The Fourier Theorems

• shift in one domain is a phase gradient in the
  other

• multiplication in one domain is convolution in
  the other

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Intensity and Visibility

• for a phase center at the pointing center xp

phase center
pointing center
primary beam
true transform of sky brightness
transform of primary beam
frequency dependent
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Support in the uv Plane

• visibility uv plane convolved with aperture
  x-cor

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Fourier Symmetries

• symmetries determined by Fourier kernel
• exp( if ) cos f i sin f
• Real ? Real Even Imag Odd
• Imag ? Real Odd Imag Even
• Real Even ? Real Even
• Real Odd ? Imag Odd
• Even ? Even Odd ? Odd
• real sky brightness ? Hermitian uv plane
• complex conjugate of visibility used for
  inverse baseline

image errors with odd symmetry or asymmetric
often due to phase errors
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Transform Pairs - 1
Figure used without permission from Bracewell
(1986). For educational purposes only, do not
distribute.
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Transform Pairs - 2
Figure used without permission from Bracewell
(1986). For educational purposes only, do not
distribute.
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Transform Pairs - 3
Figure used without permission from Bracewell
(1986). For educational purposes only, do not
distribute.
17
Transform Pairs - 4
Figure used without permission from Bracewell
(1986). For educational purposes only, do not
distribute.
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Error Diagnosis

• amplitude or phase errors
• phase errors usually asymmetric or odd symmetry
• amplitude errors usually symmetric (even)
• short duration errors
• localized in uv plane ?distributed in image
  plane
• narrow ? extended orthogonal direction in image
• long timescale errors
• ridge in uv plane ? corrugations in image
• ring in uv plane ? concentric Bessel rings in
  image

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Additive or Multiplicative?

• some errors add to visibilities
• additive in conjugate plane
• examples noise, confusion, interference,
  cross-talk

• others multiply or convolve visibilities
• multiplication ? convolution in conjugate planes
• examples sampling, primary beam, gain errors,
  atmosphere

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Antenna Based Errors

• often easier to diagnose in uv plane
• typically due to single antenna
• short duration pattern of baselines
  (six-pointed for VLA)
• long duration rings (concentric corrugations
  in image)
• effect in image plane diluted by other antennas
• factor Nbad / Ntot of baselines affected
• many antenna-based errors obey closure relations
  and are self-calibratable

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Closure Relations

• complex antenna voltage gain errors

• cancel out in special combinations of baselines

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Additive Errors

• adds to visibilities ? transform adds to image
• unconnected to real sources in the image
• may make fake sources
• sources of additive errors
• interference (sources outside beam, RFI,
  cross-talk)
• baseline-dependent errors
• noise
• short strong gain errors can appear to be
  additive

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Noise in Images

• computable from radiometer equation
• you should know expected noise level given the
  conditions
• unexpectedly high noise levels may indicate
  problems
• additive, same uv distribution as data
• will show same sidelobe pattern as real sources!
• issues
• deconvolution will modify noise characteristics
  beware!
• self-calibration on weak sources can manufacture
  a fake source from noise (especially with short
  solution intervals)

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Multiplicative Errors 1

• multiplies visibilities ? convolved with image
• will appear to be attached to sources in the
  image
• antenna gain calibration errors
• sidelobe pattern (e.g. Y for short durations,
  ripples or rings for longer durations)
• troposphere and ionosphere
• troposphere 8 GHz, water vapor content plus dry
  atmosphere
• ionosphere lt 8 GHz, total electron content (TEC)
• antenna-based errors (usually closing),
  calibratable

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Multiplicative Errors 2

• atmosphere and ionosphere (continued)
• worse on longer baselines
• short-term (sub-integration) errors smear image
• long-term large-scale structures cause image
  shifts/distortions
• phase gradient ? position shift (Fourier shift
  theorem)
• equivalent to optical seeing
• need phase referencing to calibrator(s) for
  astrometry
• but, keep track of turns of phase!
• wide-field higher-order distortions over field
  of view
• VLBI incoherent between antennas phase
  referencing needed

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Multiplicative Errors 3

• uv sampling
• sampling multiplicative with true uv distribution
• sampling function ? dirty beam convolved with
  image
• considered under Imaging Deconvolution
• primary beam and field-of-view
• multiplicative in image plane ? convolves uv
  distribution
• for baseline cross-correlation of aperture
  voltage patterns
• compact in uv plane ? extended sidelobes in image
  plane
• can be corrected for in image plane by division
  (PBCOR)
• can be compensated for in uv plane by mosaicing

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uv Coverage
snapshot coverage
8 scans over 10 hours
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Radially Dependent Errors

• not expressible as simple operations in image/uv
  plane
• sometimes convertible to standard form via
  coordinate change
• smearing effects
• bandwidth radial - like coadding images scaled
  by frequency
• time-average tangential baselines rotated in
  uv plane
• pointing
• dependent on source position in the field
• polarization effects worse (e.g. beam squint)

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Example Error - 1

• point source 2005403
• process normally
• self-cal, etc.
• introduce errors
• clean

no errors max 3.24 Jy rms 0.11 mJy
6-fold symmetric pattern due to VLA Y
13 scans over 12 hours
10 amp error all ant 1 time rms 2.0 mJy
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Example Error - 2
10 deg phase error 1 ant 1 time rms 0.49 mJy
20 amp error 1 ant 1 time rms 0.56 mJy
anti-symmetric ridges
symmetric ridges
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Example Error - 3
10 deg phase error 1 ant all times rms 2.0 mJy
20 amp error 1 ant all times rms 2.3 mJy
rings odd symmetry
rings even symmetry
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Editing Search Destroy!

• For calibrators be ruthless!
• single errors can propagate to bad solutions
  which will affect longer intervals
• may want to flag target source data around
  flagged calibrator scans
• For target sources keep in mind image-plane
  effect
• single bad integrations highly diluted in image
• long-term offsets can be more serious

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Editing What?

• plot amplitude phase versus time
• plot baselines versus a given antenna, look for
  outliers etc.
• discriminates antenna-based and baseline-based
  errors
• TVFLG (AIPS), msplot (aips), vplot (difmap)
• check different IF and polarization products
• may be best to delete all data to a given antenna
• for polarization observations, flag cross-hands
  (e.g. RL,LR) also when editing parallel hands
  (e.g. RR,LL)

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Example Edit msplot (1)
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Example Edit msplot (2)
Fourier transform of nearly symmetric planetary
disk
bad
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Example Edit TVFLG (1)
AN9 bad IF2
AN1
AN2
Jupiter structure!
time
quack these!
bad
baseline
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Example Edit TVFLG (2)
AN10 pointing?
Q-Band
AN16 low
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More on Editing

• editing tricks
• also plot versus uv distance (e.g. UVPLT,
  msplot)
• plot differences versus running mean (amplitude
  phase)
• antenna temperatures e.g. TYPLT (AIPS)
• calibration solutions SNPLT (AIPS), plotcal
  (aips)
• also check IF 1-2 and R-L for anomalous jumps
• special cases
• spectral line continuum versus line channels
• VLBI channels, delay and rate solutions
• autocorrelations

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Example Edit TVFLG (3)
AN23 problems
amplitude differences
quack these!
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Example Edit TVFLG (4)
phase differences
bad scan low amp phase noisy!
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Example Edit - SNPLT
180 deg R-L phase jump in AN13
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Interference 1

• strong additive errors
• most often seen on short baselines
• bright sources in sidelobes
• Sun (and Cyg-A at low frequencies) can be seen
  even though the source may be offset by many
  primary beams!
• watch for aliasing near map edge
• cross-talk between antennas
• short baselines, especially when shadowing
  occurs
• delete baslines where an antenna is shadowed

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Interference 2

• radio frequency interference (RFI)
• variable, often short duration bursts, sometimes
  CW
• predominantly on baselines with low fringe rate
  (e.g. N-S) and when pointed at low elevation
• usually narrow band - avoidable or excisable in
  high spectral resolution modes when isolated to a
  few channels
• system must have high degree of linearity to deal
  with strong signals without saturation

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Example RFI
The spectrum above left shows VLA 74 MHz spectral
data in the presence of a broad RFI feature.
After hanning smoothing the data with time, the
broad feature is effectively removed, leaving
only narrow easily excisable features in the plot
at the right.
Spectral plots provided by Rick Perley
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Other Problems

• wide-field imaging and mosaicing
• particularly susceptible to pointing errors and
  smearing
• high-dynamic range imaging
• all these problems will be exacerbated!
• even weak errors (especially non-closing) will
  affect data
• the most difficult type of problem
• other issues or hints
• detection experiments for weak source forgiving

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Other Suggestions

• polarization as a diagnostic
• polarization images of unpolarized sources useful
• V images sometimes useful (depends on cal scheme)
• also, look at differences between IFs or channels
• low-resolution images good first step
• image full field-of-view and sidelobes
• find confusing sources or signature of RFI
• FT errors in image back to uv plane
• identify source (antennas, baselines, or
  integrations)

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Summary

• effect in image depends on effect in uv plane or
  time stream
• understand the properties of the Fourier
  transform
• errors may be additive, multiplicative, radially
  dependent
• move between image and uv plane
• effective editing
• know expected noise levels, gauge severity in
  image
• edit calibration scans carefully
• image the full beam to look for confusing sources
  or RFI
• use visualization tools (getting better all the
  time)
• know the effects of the imaging calibration
  algorithms

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Extra - Snapshot Sequence

• imaging and self-calibrating VLA snapshot
• A-config, 8.4 GHz, 30 sec, 0.2 resolution,
  0.4mJy rms
• processed in difmap
• special problems for snapshots
• poor uv coverage high sidelobes
• weak source, must be careful in self-calibration
• modelfitting instead of cleaning see talk by
  Pearson
• point source nature makes this easier!

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Snapshot 1
uv coverage
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Snapshot 2
amplitude vs. uv radius
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Snapshot 3
dirty beam
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Snapshot 4
dirty image - low resolution
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Snapshot 5
dirty image - full resolution around peak
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Snapshot 6
residual image - 1st source removed
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Snapshot 7
residual image - 2nd source removed
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Snapshot 8
residual image - 3rd source removed
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Snapshot 9
residual image - 4th source removed
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Snapshot 10
residual image - phase self-cal
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Snapshot 11
final restored image 4 image gravitational lens!